Field of the Invention
The present invention generally relates to a sequential differential mobility analyzer for separating and concentrating the size of selected target ions or charged target particles (collectively “Target Particles”). More particularly, the invention disclosed herein primarily uses a combination of differential aerodynamic mobility and differential lateral electrical mobility, within sequential regions having both airflow(s) and electrical field(s), to separate Target Particles (having a targeted size and electronic charge) from other particles.
A differential mobility analyzer (“DMA”) is an instrument typically used to separate small charged aerosol particles based on their electrical mobility, for detection and classification. Many DMAs include two charged concentric cylindrical electrodes, creating an electric field between adjacent electrode walls. This essentially annular pathway (or annular region) between adjacent electrodes may be considered the analysis region. Also included is an aerosol inlet for introducing sample particles (including Target Particles) into the instrument. A sheath gas inlet permits sheath gas (or sheath gas, collectively “sheath gas”) to flow into the instrument between the electrodes, which draws the polydispersed particles through the annular region.
In most cases, the resolution of the DMA is limited by diffusion, turbulence, initial spatial distribution of particles, and the ratio of aerosol flow to sheath flow which relates to the transfer function of the particles.
One disadvantage with using only two electrodes is that traditionally there is only one drift region of electrical field inducing differential lateral drift of different particles due to each particle's electrical charge and aerodynamic diameter.
Another disadvantage with using traditional tandem or sequential DMAs is that they do not include a plurality of analysis regions within the same housing, and fewer target particles will be separated en route to the instrument exit. The typical sequential DMA setup will not materially increase the resolution above that of its individual DMA components.
Yet another disadvantage of a regular DMA is that sheath flow through the gap in electrodes is directed inwardly toward the central exit. This is required to improve the particle transport efficiencies only because classified aerosol flow which contains only target particles are suctioned by the external pump. This flow direction reduces the resolving power of the instrument because it does not prevent the diffusive crossing of unwanted particles (including neutral particles).
Another disadvantage of existing DMAs is that target particles are diluted because the classified aerosol flow rate is high or fast, to achieve sufficient transport efficiencies. Consequently, possible coupling devices such as a mass spectrometer cannot utilize all the particles in the classified aerosol flow. In this regard, the detection efficiencies are severely limited.
Background of the Invention
The electrical mobility of a charged particle is inversely related to the particle's size; smaller particles exhibit greater mobility within an electrical field than do larger particles (of like charge). Conversely, larger particles travel more in a “downwind” direction during its longer residence time in the drift region due to their smaller electrical mobilities. By calibrating and coordinating the parameters of both the airflow and the gradient(s) of the electrical field(s) transversing the airflow route(s), smaller-than-targeted particles can be electronically attracted while larger-than-targeted particles continue being swept downstream with the airflow, so that only the Target Particles exit the instrument. Ideally only the Target Particles, having the desired electrical mobility and particle size, are extracted from the analyzer.
The following patents are arguably material to the patentability of the invention disclosed herein:
patent/Date of Issue/application No.1st InventorPublication6,607,597Sun et al.Aug. 19, 20036,787,763De La Mora et al.Sep. 7, 20047,161,143De La Mora et al.Jan. 9, 20077,213,476Cheng et al.May 8, 20077,521,673Areas et al.Apr. 21, 20097,723,677Ramiro Areas et al.May 25, 2010
U.S. Pat. No. 7,723,677 issued to Ramiro Arcas et al. essentially discloses a DMA having an electric field component opposite to the drag flow to cause the main electric field to be oblique to the velocity field of the drag flow, rather than perpendicular to the velocity field of the drag flow. It discloses a control volume with a rectangular base in which two opposing walls made up of electrodes define an electric field. The two remaining opposite sides of the region form an inlet and outlet of the ordinary cross flow, which is perpendicular to the electrodes. It also discloses the usage of resistive electrodes or conductive electrodes separated by insulators to achieve an electric field against the sheath flow inside the controlled volume. With the external circuit being open or closed, the controlled volume can be switched from classic DMA to DMA utilizing oblique fields against the sheath flow. The device contains shared controlled volume as well as a single inlet with multiple exit slits. One of the exit slits located upstream is used when the device is used as DMA with oblique field.
U.S. Pat. No. 7,213,476 issued to Cheng et al. essentially discloses a multi-stage DMA for aerosol measurements including a first electrode having at least one inlet for receiving an aerosol including charged particles for analysis. A second electrode is spaced apart from the first electrode, and has at least one sampling outlet disposed at a plurality of different distances along its length. A volume between the first and second electrode between the inlet and one of the outlets forms a classifying region, with the first and second electrodes for charging to suitable potentials to create an electric field within the classifying region. The inlet in the first electrode receives a sheath gas flow at an upstream end of the classifying region, wherein each sampling outlet functions as an independent DMA stage and simultaneously classifies different size ranges of charged particles based on electric mobility. The aerosol is preferably injected from a central electrode and the sampling flow is preferably withdrawn through an outer electrode.
None of the cited patents expressly disclose a sequential DMA analyzer having a housing enclosing electrodes forming a plurality of sequential DMA analysis regions without overlap of controlled volume for analyzing a Target Particle, with the sample aerosol intended to initially travel downstream with the sheath flow without pump assistance, and including a plurality of guide electrodes for guiding Target Particles to the exit outlet.